1. Field of the Invention
The present invention relates to a process and an apparatus for manufacturing MOS devices from semiconductor (e.g. silicon) wafers.
2. Description of the Related Art
The thermal oxidation is one of processes for growing a silicon oxide film on a surface of a silicon wafer. For example, the field oxide film growth of the thermal oxidation employs the so-called high-temperature oxidation in which an oxidizing agent, such as oxygen (O.sub.2) or a steam (H.sub.2 O), is fed into a quartz tube and heated at the high temperature of 1100.degree. C. to grow a silicon oxide on the surface of the silicon wafer. Recently, a thin oxide film such as a gate oxide occasionally employs an oxidation at a low temperature of 800.degree.-900.degree. C. and/or in the diluted oxidizing gas reducing an oxidation rate in order to unify the in-plane thickness of the oxide film on the silicon wafer.
Oxide films grown by such processes include electric charges such as the fixed charge. It is known that these charges vary the surface potential of the silicon and especially cause important problems in the yield and the reliability of a MOS device. Hitherto, many researches have been on the oxide charges (see IEEE. TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-27, NO.3, MARCH 1980, pp.606-608).
It has been known that the charge density decreases when an oxidation temperature increases and the charge density further decreases when the oxidized silicon wafer is annealed in an ambient of an inert gas such as argon or nitrogen (see J. Electrochem. Soc: SOLID-STATE SCIENCE AND TECHNOLOGY, March 1967, pp.266-273). It has been known in a subsequent research that the charge density increases by the presence of dry oxygen in the inert gas (see J. Electrochem. Soc: SOLID-STATE SCIENCE AND TECHNOLOGY, September 1971, pp.1463-1468).
The present inventor discovered in a test that fixed-charge densities are not equal when silicon oxide films are grown on silicon wafers in cylindrical tubes of heat treating furnaces with different bores under the same heat treatment conditions.
In the test, 27 silicon wafers sliced from a silicon single crystal grown by the Czochralski method were prepared. Each silicon wafer was a 4-inch diameter, the &lt;100&gt; orientation, the p-type conduction and a 10-.OMEGA.cm resistivity. All of the silicon wafers were cleaned and dried up. Subsequently, respective nine sets of three silicon wafers were put in nine horizontal heat treating furnaces A, B, C, D, E, F, G, H and I with different bores. Each set of three silicon wafers were placed at 1000.degree. C. in the ambient of a wet oxygen at a flow rate of 5 l/min for 1.5 hr and a gate oxide of a 500-nm thickness was grown on each silicon wafer. Successively, these three silicon wafers were annealed in the ambient of nitrogen at a flow rate of 5 l/min for 1 hr, respectively. Subsequently, an aluminum electrode was deposited on the gate oxide using shadow mask by a vacuum evaporator. Subsequently, the oxide of back surface was removed. Subsequently, in order to reduce the interface trap density, all the silicon wafers were heat treated at 400.degree. C. in the ambient of a mixture gas of 3% hydrogen and 97% nitrogen for 30 min. Thus, many MOS capacitors with 1.81 mm.sup.2 gate area were fabricated. The capacitance versus voltage (C-V) characteristics of each MOS capacitor were measured and fixed-charge densities at six points of that MOS capacitor of each silicon wafer were obtained. An average of the fixed-charge densities at the six points was determined as the data of each silicon wafer.
FIG. 6 shows a relation between nine horizontal heat treating furnaces with different bores and fixed-charge densities. As seen in FIG. 6, fixed-charge densities are not equal among the nine furnaces under the same heat treatment condition.
It will become greatly important that the charge density is controled in order to fabricate a microscopic device, a high density assembly device and a high speed device of integrated circuits. To this end, it will be necessary for the controlling technique not only to reduce the fixed-charge density but also to apply commonly for the heat treating furnaces with different bores.
In view of the requirements described above, the inventor variously tested under different heat treatment conditions using heat treating furnaces with different bores and discovered that the fixed-charge density strongly depends on the flow rate of an ambient gas. In a subsequent test, the inventor also discovered that the fixed-charge density depended on the clearance linear speed (CLS) which was defined as a ratio of the flow rate of an ambient gas to the area of a clearance between a silicon wafer and the interior surface of the tube, as seen in FIG. 2. Moreover, the inventor discovered that, when the CLS was constant, fixed-charge densities were substantially equal even though silicon oxide films were grown by the heat treating furnaces with different bores.